Snail, Slug and Sip1 regulate cadherin and protease expression and mediate epithelial-mesenchymal transition in cancer. We analyzed the expression of cadherins and matrix metalloproteinases (MMP) and their transcriptional regulators in malignant mesothelioma (MM). One hundred and ten MM specimens (86 solid, 24 effusions) and 10 non-malignant effusions with reactive mesothelial cells (RMC) were analyzed for E-cadherin, N-cadherin and P-cadherin protein expression using immunhistochemistry. MM effusions were further analyzed for expression of Snail, Slug, Sip1, E-cadherin, MMP-2, MMP-9, MT1-MMP (MMP-14) and the MMP inhibitor TIMP-2, and for MMP-2 and MMP-9 activity using RT-PCR, Western blotting, immunhistochemistry and zymography. Results were analyzed for relationship with specimen type (biopsy versus effusion) and anatomic site (pleural versus peritoneal). E-cadherin, N-cadherin and P-cadherin expression was found in 69/110 (63%), 87/110 (79%) and 84/110 (76%) MM cases, respectively. Pleural and peritoneal MM showed comparable expression, but all three cadherins were upregulated in effusions compared to solid tumors (p<0.001). RMC were uniformly negative for E-cadherin and N-cadherin, and showed P-cadherin expression in 7/10 specimens. Immunohistochemistry localized MMP-2, MMP-9 and TIMP-2 to MM cells in 11/15, 14/15 and 8/15 effusions, respectively. RT-PCR showed direct association between MMP-2 mRNA expression level and the levels of MT1-MMP (p=0.027) and TIMP-2 (p=0.011). Snail protein expression showed positive association with MT1-MMP (p=0.016) and TIMP-2 (p=0.02) mRNA expression, but its expression was unrelated to MMP-2 and MMP-9 expression or activity. Snail, Slug and Sip1 levels did not show inverse association with E-cadherin levels. Our data show that E-cadherin and N-cadherin are selectively expressed in malignant mesothelial cells, and that P-cadherin and N-cadherin are expressed with similar frequency in MM. In agreement with our earlier data for ovarian carcinoma, cadherin expression is upregulated in effusions compared to solid lesions. The increased E-cadherin expression in effusions may be related to lack of negative regulation at the epigenetic level. The relationship between Snail and MMP in MM is uncertain at present.